1. Field of the Invention
The present invention relates to an improved catalyst structure for use in the partial oxidation of hydrocarbons to prepare dicarboxylic acids and anhydrides.
2. Prior Art
Basically, all of the methods for producing catalysts for maleic anhydride production employ vanadium in a valence state of less than +5. One method of achieving this is to begin with vanadium in less than the +5 valence state. Another method and that used most widely in the art is to start with vanadium in the +5 state and reduce the valency to less than +5.
Usually the reduced vanadium has been obtained by reducing V.sub.2 O.sub.5 in a solution with HCl to obtain vanadyl chloride. A typical catalyst preparation may involve dissolving the vanadium, phosphorus, and other components in a common solvent, such as hot hydrochloric acid and thereafter depositing the solution onto a carrier. The reduced vanadium with a valence of less than 5 is obtained by initially using a vanadium compound with a valence of plus 5 such as V.sub.2 O.sub.5 and thereafter reducing to the lower valence with, for example, hydrochloric acid during the catalyst preparation to form the vanadium oxysalt, vanadyl chloride, in situ. The vanadium compound is dissolved in a reducing solvent, such as hydrochloric acid, which solvent functions not only to form a solvent for the reaction, but also to reduce the valence of the vanadium compound to a valence of less than 5. For example, a vanadium compound, a copper compound, a tellurium compound, phosphorus compound and alkali metal compound may be dissolved in any order in a suitable reducing solvent and the formation of the complex allowed to take place. Preferably, the vanadium compound is first dissolved in the solvent and thereafter the phosphorus, copper, tellurium and other metal compounds, if any, are added. The reaction to form the complex may be accelerated by the application of heat. The deep blue color of the solution shows the vanadium has an average valence of less than 5. The complex formed is then, without a precipitation step, deposited as a solution onto a carrier and dried. In this procedure, the vanadium has an average valence of less than plus 5, such as about plus 4, at the time it is deposited onto the carrier or precipitated without the carrier. Generally, the average valence of the vanadium will be between about plus 2.5 and 4.6 at the time of the precipitation.
In another method the catalyst is prepared by precipitating the metal compounds, either with or without a carrier, from a colloidal dispersion of the ingredients in an inert liquid. In some instances the catalyst may be deposited as molten metal compounds onto a carrier; however, care must be taken not to vaporize off any of the ingredients such as phosphorus. The catalysts have also been prepared by heating and mixing anhydrous forms of phosphorus acids with vanadium compounds, copper compounds, Me compounds, and the alkali metal compound. The catalysts may be used as either fluid bed or fixed bed catalysts. In any of the methods of preparation, heat may be applied to accelerate the formation of the complex.
A very old and traditional method of obtaining vanadyl chloride as disclosed by Koppel et al, Zeit. anorg. Chem, 45, p. 346-351, 1905 is the reduction of V.sub.2 O.sub.5 in alcoholic HCl solution. This method has been recommended for the preparation of the phosphorus-vanadium oxidation catalyst for example, by Kerr in U.S. Pat. No. 3,255,211 where the solvent also serves as the reducing agent. Subsequently, U.S. Pat. No. 4,043,943 employed this method of reducing vanadium to prepare the basic phosphorus-vanadium catalyst, however, catalyst produced in this manner are known to require a very specific activation procedure in order to be useful as catalyst, as described for example, in U.S. Pat. No. 4,017,521.
In an early series of commonly owned patents, a unique group of vanadium-phosphorus, oxidation catalysts, were disclosed, i.e., U.S. Pat. Nos. 3,156,705; 3,156,706; 3,255,211; 3,255,212; 3,255,213; 3,288,721; 3,351,565; 3,366,648; 3,385,796 and 3,484,384. These processes and catalysts proved highly efficient in the oxidation of n-butenes to maleic anhydride. Since the issuance of these pioneer patents, numerous patents have issued with various modifications and improvements over the basic discoveries set forth there, e.g., U.S. Pat. Nos. 3,856,824; 3,862,146; 3,864,280; 3,867,411; 3,888,886; 4,071,539; 4,097,498; 4,105,586; 4,152,338; 4,152,339 and 4,153,577.
In a recently developed procedure disclosed in the commonly assigned U.S. Patent application Ser. No. 047,323 filed June 11, 1979 which is incorporated herein in its entirety, an improved catalyst is that produced from an alcoholic HCl solution reduction of vanadium pentoxide wherein the organic solvent is an alcohol such as isobutyl alcohol and the reduction of the vanadium is obtained by contacting it with HCl. This is conveniently carried out by passing gaseous HCl through the alcohol having the vanadium pentoxide suspended therein. The vanadium pentoxide is reduced by the HCl and brought into solution as the vanadyl chloride. The completion of the reduction is the appearance of a dark reddish brown solution. Hydrogen bromide would be about the same as a reducing agent in this system. It has been found that the reduction temperature should be maintained at no greater than 60.degree. C. and preferably less than 55.degree. C. Optimumly active catalysts are the result when the reduction is carried out at temperatures in the range of about 35.degree. C. to 55.degree. C., preferably 40.degree. to 55.degree. C.
To obtain the mixed oxides of vanadium and phosphorus, phosphoric acid of approximately 99%, H.sub.3 PO.sub.4 (98 to 101%) is added, for example, prepared from 85 H.sub.3 PO.sub.4 and P.sub.2 O.sub.5 or commercial grades of 105 and 115% phosphoric acid diluted with 85% H.sub.3 PO.sub.4 and the vanadium compound digested, which is discerned by a change in the color of the solution of a dark blue green. Zinc or other catalyst components are conveniently added along with the phosphoric acid. The alcohol is then stripped off to obtain the dried catalyst.
Catalysts have been prepared in various shapes and configurations, for example, saddles, discs, spheres, cylinders, tubes, granules and the like. For example, U.S. Pat. No. 2,078,945 discloses hydrosilicate catalyst may be formed in tubes or solid cylinders, which may then be crushed and screened to provide irregular catalyst shape. U.S. Pat. Nos. 4,178,298 and 4,181,628 both disclose that mixed oxide oxidation catalyst containing vanadium and phosphorus may be employed as pellets, tablets or cylinders. Rounded aggegate having a void center and a single cavity in the external surface communicating to the void center and named amphora is described in U.S. Pat. Nos. 3,848,033; 3,966,639; 4,094,922 and 4,171,454. U.S. Pat. Nos. 4,153,539 and 4,170,569 disclose rounded similar catalysts which has been named amphora II, but having two cavities 180.degree. C. opposed communicating with the hollow center. The amphora II aggregate is described as particularly effective and advantageous in any process in which the feed is present in the reactor partially in the liquid phase.
The production of dicarboxylic acid anhydride by catalytic oxidation of hydrocarbons is well known. The current principal route for the production of maleic anhydride from C.sub.4 hydrocarbons has been desirable in the past, but is now even more desirable in view of the particular world shortage of benzene. It can be readily appreciated that direct oxidation of C.sub.4 hydrocarbons would be a hydrocarbon conservation, since for each mol of maleic anhydride prepared from benzene, one mol of benzene, molecular weight 78 is consumed, whereas for each mol of the C.sub.4, only 54 to 58 mol weight of hydrocarbon is consumed. The benzene process has consistently produced high conversions and selectivities.
A more desirable process for producing maleic anhydride would be a direct oxidation of n-butenes or butadiene. Also, n-butenes may have higher economic petrochemical utilization than the n-butanes, which are now, often wastefully burned as cheap fuel.
Normal butane requires a solid tableted catalyst rather than a support with the catalytic component deposited thereon because of the energy requirements. Due to the high loading necessitated by economics, conversions of butane in excess of 75% have been unobtainable. Higher conversion results in high hot spot temperatures which adversely effect yield.
It is an advantage of the present invention that greater catalyst activity is obtained. It is a further advantage that less weight of catalyst is employed. Another advantage is that lower pressure drop through the reactor is obtained. It is a feature that the reduced pressure drop allows for high flow rates and increased production results. A further advantage is better heat removal from the reactor zone, which allows for higher conversions and greater productivity. These and other advantages and features will become clear from the following description and discussion.